I am not even sure how it would work in software mode. How would you know which buffer you are starting to draw to, the left or right if it's simply done on timing. Surely you'd need some hardware solution to sync to the display.

As the title suggests. I was wondering if this is possible simply by rendering at 120hz.

So every odd frame we render to the right-eye, and every even frame we render to the left eye?

Method of left/right frame determination varies significantly between display models. Couple of months ago I've seen an open-source library, which does all the job of interlacing/concatenating right and left images for any of stereo3d mods in one fragment shader pass. Sorry, cant find it in google now